Animation Nodes - How do I read the Particle Count with the Object Attribute Output node?
The full python line to read that setting is:

bpy.data.objects['Plane'].particle_systems['ParticleSystem'].settings.count


According to the Animation Nodes documentation the attribute field of the Object Attribute Output node is relative to the object which would mean you would use:

particle_systems['ParticleSystem'].settings.count


or alternatively:

modifiers['ParticleSystem'].particle_system.settings.count


Whether this actually works is dependant on how the developer wrote the Animation Nodes addon.